The present invention relates to an optical distance detecting device employing image sensors, particularly to an inter-vehicle distance detecting device which continuously measures an inter-vehicle distance from a driving vehicle to a preceding vehicle.
Conventionally, optical distance detecting devices employing image sensors are disclosed in Japanese Examined Patent Publication No. 38505/1988, Japanese Examined Patent Publication No. 46363/1988 and the like. All of these devices are provided with two left and right optical systems, as shown in FIG. 15. These left and right optical systems are constituted by lenses 1 and 2 disposed spaced apart from each other by a base length L.
Separate image sensors 3 and 4 are provided at positions spaced apart from the lenses 1 and 2 by a focal length f. In an image processing device 51, image signals of the image sensors 3 and 4 are electrically superposed by being successively shifted. A distance R from the lenses to an object 52 is provided by the following equation (1), from a shift quantity n wherein the above two images signals fit the most, under a principle of trigonometry. EQU R=f.times.L/n (1)
On the other hand, a method of image-tracking an image of a preceding vehicle taken by image sensors or the like, is disclosed in Japanese Examined Patent Publication No. 33352/1985. According to the publication, a target is image-tracked by setting a tracking gate (window) surrounding the target to be tracked on a display image plane, while an operator is observing the display image plane.
Since, in the conventional distance detecting device, the distance to the object is provided by comparing the images taken by the pair of left and right optical systems, in case wherein an inter-vehicle distance from a driving vehicle to a preceding vehicle is measured by mounting the device on the driving vehicle, when another vehicle is running on the lefthand side or on the righthand side of the preceding vehicle, a driver is not informed of which inter-vehicle distance of vehicle the device is measuring.
Furthermore, concerning a method of image-tracking an image of a preceding vehicle taken by image sensors or the like, the conventional image-tracking device is constructed such that the target is image-tracked by setting a gate (window) surrounding the object to be image-tracked on a display image plane, while an operator is observing the image display plane. The device is provided with a function wherein the device continues tracking a target once the target is determined, so far as the target is in the scope of vision even if it is moving. However, in an actual road, there are cases wherein another vehicle cuts in between the driving vehicle and the preceding vehicle, or the preceding vehicle image-tracked currently changes the driving lane to another lane. When this situation takes place, the vehicle which has been regarded as the former preceding vehicle is not the currently tracking "preceding vehicle", and the cut-in vehicle or a vehicle in front of the former preceding vehicle becomes a new "preceding vehicle". In the conventional tracking device, once the image of the preceding vehicle is set as the target on the display image plane, so far as the image of the preceding vehicle is present on the display image plane, it is possible to image-track the image of the preceding vehicle. However, when a new preceding vehicle shows up, it is not possible to detect the showing-up and reset the new preceding vehicle as the target on the display image plane. Accordingly, the driver must drive the vehicle while always observing the display image plane in details, and manually reset the tracking window every time when the cutting-in or the lane change takes place, which is a very dangerous driving.